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unable to merge selected threads

When you want to merge face zones inside fluent, sometimes it gives the following error message:

unable to merge selected threads

Thread and face zone are just two different names for the same thing. The reason why it fails is that the two face zones you selected are NOT adjacent to the same cell zone. The way to check is as follows:

1) Change the face zone boundary type to wall
2) Go to define boundary panel and it tells you the adjacent cell zone. If your type is not wall, then the adjacent cell zone will not show up and that's why step 1) is there.

Note that fluent does not allow one face zone to touch two cell zones. Actually, if in your preprocessor you have one face zone touching two cells zones, inside fluent this face zone will be broken up into facezone and facezone:001. The solution to the problem is to merge the cell zone before you merge the face zone.